With the continued improvement of the continuous casting—hot rolling process, hot rolling slabs are changed from original blooms to continuous-casting slabs. Usually, over 90% of hot rolling slabs come from the continuous casting.
During continuous casting, molten steel is poured, solidified and cut, and after this, the cut continuous-casting slab is sent into the hot rolling line to be rolled. Currently, the method of cutting a continuous-casting slab into cuboids is used internationally.
A conventional hot continuous rolling production line consists of a heating furnace, a rough rolling equipment, a finishing rolling equipment, a laminar cooling equipment and a coiler equipment, wherein in the region of the rough rolling equipment, there are provided with a roll table, a descaling machine, a slab fixed width press, a rough mill, a measuring meter and the like. Usually, the rough mill consists of a horizontal mill and an auxiliary edger mill, and it can perform rolling reversibly, so as to reduce the thickness or width of the slab. A typical layout of rolling line equipments is shown in FIG. 1.
The temperature drop during hot rolling has a substantial impact on the material properties and the rolling stability. For guaranteeing the rolling temperature during the finishing rolling, the whole line has to manufacture with the minimum passes and the highest speed so as to reduce the heat loss. If processing times in a device is an even number, there must be one dummy pass, which may result in a meaningless temperature drop. For reducing the temperature drop of material as much as possible, the process times in a rolling device must be always an odd number. For a production line configured with two rough mills (R1, R2), the passes of R1/R2 may be 1/5, 3/3 and the like.
Due to the odd characteristic of the process and the effect of the edger rolling, the deformation of the head and the tail of the material is asymmetric, which, in turn, causes the asymmetry between the shapes of the processed head and tail. A typical cuboid slab after processing by rough rolling equipments may be formed into an intermediate slab with a fishhead and a dovetail, as shown in FIG. 2.
During finishing rolling of hot rolling thin strip steels, a high speed rolling technique is utilized to improve the utilization efficiency of the equipments and reduce the temperature drop. The irregular shapes of the head and the tail of the intermediate slab after rough rolling may cause accidents when the material enters into the finishing mill, for instance, the head cannot pass through rollers smoothly, or the tail cannot be rolled stably. In view of this, a set of flying shear is provided between the rough mill and the finishing mill to cut away the irregular parts of the head and tail of the intermediate slab, which may cause the yield loss during production, affecting the production efficiency of the hot rolling line. Empirically, the loss for cutting the head and tail of the intermediate slab accounts for about 30% of the hot rolling yield loss. Assuming that the length of the intermediate slab is 60 m, and the head and the tail are cut by 150 mm respectively, the total cut amount is 300 mm, accounting for 0.5% the whole material. Therefore, improving the shapes of the head and tail and decreasing the cut amount thereof is one significant subject for iron & steel enterprises.
To improve the shapes of head and tail of the intermediate slab after rough rolling and increase the yield of the hot rolling line, the skilled have developed a lot of equipments and control techniques. For instance, large side press equipments for slab are used for pressing in width, and the pass of vertical rollers in the rough rolling region is controlled by using short head and tail stroke control method, so as to improve the shape of the head and tail. But even if a variety of methods have been used, the yield loss resulted from the bad head and tail shapes of the intermediate slab is still a main problem.